The crash occurred on the new Sheppey crossing bridge on the A249 in Kent at 7.15am and emergency services are dealing with the incident
A Kent Police spokeswoman said visibility was a problem over the bridge, but it was not clear whether that caused the pile-up. Motorists were warned to avoid the area. There are reports of six serious injuries and 200 minor injuries, police said.
"Kent Police was called at about 7.15am on Thursday September 5 to the London-bound carriageway on the Sheppey crossing," a force spokesman said. "It has been reported there were collisions at the top of the crossing and at the foot of the approach to the crossing. Emergency services are currently at the scene dealing with the incident.
The accident happened in thick fog with some witnesses saying that visibility was down to 20 yards in places and some motorists were driving like idiots prior to the pile-up.
The scene on the bridge was a mass of tangled cars, lorries and even a car transporter, with reports saying that the crash went on for 10 minutes as cars continuously collided with each other. The uninjured and the walking wounded are being taken down from the bridge on the Sittingbourne side.
Witness Martin Stammers, 45, from Minster, told Kent Online: "It's horrific. I've never seen anything like it in my life. All you could hear was cars crashing. We got out of our car and it was eerily quiet, with visibility down to just 20 yards."
A Kent Fire and Rescue Service spokesman also said there were no deaths. A spokesman said: "Crews are currently at the scene of a serious RTC (road traffic collision) involving up to 100 vehicles. There are no fatalities but ambulance crews are dealing with a large number of walking wounded casualties. Firefighters have used hydraulic cutting equipment to release five people from their vehicles."